I have an inline water heater. Once it gets hot it will stay that way for 20+ minutes at a time easily.
When the shower gets used it feels like their is a floor in how cold the water will get - if I turn the cold up past a certain point it runs ice cold.
This waterheater has been in a while. I've already flushed the lines with vinegar and tripple checked the gas and gas regulator.
What more should I look at? My guess would be the water flow control mechanism but I really don't know.